Orthogonal Frequency Division Multiplexing (OFDM) technology is widely applied in the next generation wireless communication system like Long Term Evolution (LTE), because of the advantage of high frequency spectrum efficiency and eliminating multipath fading effectively. The principle of the OFDM is to divide a broadband transmission channel into a series of parallel narrowband sub-channels whose frequency spectrums overlaps mutually. In order to keep the orthogonality between sub-channels and avoid the mutual interference between sub-channels, the requirements of OFDM technology for synchronization of time and frequency are high.
In the uplink channel of LTE, the base station receives transmission signals from different user equipment (UE), and the time when the transmission signals from UE arrives at the base station should be the same, so avoid the mutual interference between the signals of different UE. The distances between different UEs and the base station being different, this requires different UEs transmitting signals at different time, so that the time when the transmission of signal from the UE that is farther away from the base station is relatively earlier. While the UE is unable to measure the distance from the base station, the corresponding technology applied in the LTE is: in the case that the UE accesses the system for the first time or UE can't keep the synchronization with the base station, the physical random access channel (PRACH) signal is transmitted to the base station, and the base station detects the time when the PRACH signal arrives and feedback the time advance (TA) of the transmission time required by the UE to the UE for corresponding adjustment.
The PRACH signal in the LTE is based on the ZC (Zadoff-Chu) sequence, a ZC sequence with the length of NZC and base of u is:xu,k=e−jπuk(k+1)/NZC, 0≦k≦NZC−1
It is because of the following characters that the ZC sequence is chosen:
for the same base, the ZC sequences generated by different cyclic shifts are orthogonal;
for different bases, the correlation value between the ZC sequences is constant.
After receiving the PRACH signal, the base station tries carrying out the correlation operation of the received signal and the PRACH signal at different time by using different ZC sequences. If the obtained correlation value is greater than a certain threshold, it is believed that the transmitted PRACH signal is detected, and the time when the PRACH signal arrives at the base station is obtained at the same time, that is, the transmission delay from the UE to the base station is obtained.
In the wireless communication channel, the transmission signal is transmitted to the base station by different paths which have different lengths. Therefore, what the base station receives is a series of PRACH signals with different time delays. Since a series of PRACH signals are orthogonal, the base station detects the PRACH signals with greater energy. This will lead to unfavorable results as follows:
1. The energy of the transmission signal is dispersed in every path, which decreases the probability of the base station detecting the PRACH signals;
2. Since multiple paths are detected, the base station could not judge the transmission delay accurately from the UE to the base station.